1. Field of the Invention
The present invention relates, in general, to an electrochemical machining process and, more particularly, to an electrochemical machining process capable of fabricating a cylindrical microprobe having a uniform diameter throughout its entire length by properly controlling an applied electric field.
2. Description of the Prior Art
Electrochemical machining, also known as electrolytic machining, is a technique to remove excess metal by electrolytic dissolution, effected by the tool acting as the cathode against the workpiece acting as the anode. That is, a chemical reaction occurs between the workpiece and the tool, both being beneath the surface of an electrolyte, under an electric field to dissolve the workpiece into the electrolyte. Usually, the electrochemical machining procedure is carried out through the following four steps: migration of ions of the electrolyte to the surface of the electrodes; reaction of metal atoms on the workpiece with the ions to form molecules; conversion of the molecules into stable ions; and diffusion of the stable ions into the electrolyte.
On the whole, the characteristics of the electrochemical machining are determined by the rates of the four steps. For instance, where the rate at which metal atoms on the workpiece react with ions of the electrolyte to form molecules is greater than the rate at which the molecules are converted into stable ions, the electrochemical machining characteristic is of electrolytic polishing, that is, the final appearance of the metal surface is smoothed and enhanced by making it an anode in the electrolyte during the electrochemical machining procedure. On the other hand, in the reverse case, the electrochemical machining characteristic is of electrolytic etching, that is, the surface of the metal is engraved by electrolysis during the electrochemical machining procedure. Consequently, differences in rate among the four steps of the electrochemical machining procedure have a decisive influence on the appearance and shape of the workpiece. Typically, the dissolution rate of the workpiece is determined by the fourth step, the diffusion of the ions into the electrolyte.
Electrolytic etching is usually used to fabricate fine probes with a degree of precision of several nanometers. In this regard, this process is conducted in relatively low concentrations of electrolytes under a relatively weak electric field. During the electrolytic etching, the workpiece typically undergoes the dissolution at a faster rate on its sites which have large curvatures, e.g., end portions, than on lengthwise side portions, thus being transformed into an asymptotic cone. This phenomenon is called a geometric effect.
Problems with such conventional electrolytic etching are as follows.
Because of their being located at different depths from the surface of the electrolyte, portions of the workpiece are subjected to different machining conditions, which cause inconstant local dissolution rates. Accordingly, uniformly shaped-workpieces with ultrafine diameters are difficult to produce with such conventional electrolytic etching. Additionally, such different local dissolution rates make it difficult to precisely process the workpiece into various shapes.
With the problems in mind, the present invention has an object of providing an electrochemical machining process for fabricating cylindrical microprobes having uniform diameters throughout their entire lengths.
It is another object of the present invention to provide an electrochemical machining process for fabricating cylindrical microprobes into various shapes with precision.
Based on the present invention, the above objects could be accomplished by a provision of an electrochemical machining process for the fabrication of a cylindrical microprobe, comprising: a preparatory step for measuring a contact point through which an electrical current is first flowed when a workpiece set as an anode is brought into the electrolyte after immersing a cathodic tool in the electrolyte and for dipping the workpiece in an electrolyte to the length to be processed on the basis of the measured contact point after removal of the applied electric field; a condition-setting step for setting the diameter to be processed of the workpiece, the electrochemical equivalent volume constant of the workpiece, the current density to be applied across the electrodes, and the machining time interval for which to apply the current; a machining step for electrochemically machining the workpiece while continuously calculating and measuring changes in the surface area of the workpiece, the electric current flowing through the electrolyte, the quantity of electricity applied, and the diameter of the workpiece with machining time; a finishing step for determining whether or not the diameter of the workpiece reaches the preset value or whether the machining step is required to be repeated to further approximate the diameter of the workpiece to the preset value, and stopping the machining step if a desired value is obtained from the measured diameter of the workpiece.
As the electrochemical machining process proceeds, the diameter of the workpiece is changed as calculated according to the following equation:
Am=xcfx80[LD+H(Do+2D)/3]
wherein, Am is the surface area of the workpiece, which changes as the electrochemical machining process proceeds, represented in mm2; L is the length of the workpiece""s portion to be processed, represented in mm; D is the diameter of the workpiece""s portion processed, which changes as the electrochemical machining process proceeds, represented in mm; and Do is the original diameter of the workpiece""s portion to be processed, represented in mm.
During the electrochemical machining process, the electric current to be applied across the electrodes is calculated according to the following equation:
i=AmJ
wherein, i is a current applied per time, represented in C/sec; Am is the surface area of the workpiece""s portion processed, which charges as the electrochemical machining process proceeds, represented in mm2; and J is a current density, represented in C/mm2 sec.
As for the quantity of electricity applied, it is controlled according to the following equation:
Qt=Qp+ixcex94t
wherein, Qt is the total quantity of electricity applied for entire machining time period, represented in C; Qp is the quantity of electricity used in the previous machining round, represented in C; and xcex94t is the electrochemical machining period of time, represented in sec.
The diameter of the workpiece processed changes as the electrochemical machining process proceeds and is calculated from the following equation:
xcfx80(Doxe2x88x92D)[L(Do+D)/4+h(3Do+2D)/15]/ae=Qt
wherein, D is the diameter of the workpiece, which changes as the electrochemical machining process proceeds, represented in mm; Do is the original diameter of the workpiece""s portion to be processed, represented in mm; Qt is the total quantity of electricity applied for entire machining time period, represented in C; L is the length of the workpiece""s portion to be processed, represented in mm; h is the practical length of the workpiece which is in contact with the electrolyte owing to the surface tension, represented in mm; and ae is an electrochemical equivalent volume constant of the workpiece.
In an version of the present invention, the machining step is carried out in such a way that metal ions on the surface of immersed portions of the workpiece are controllably dissolved and diffused into the electrolyte through the application of an electric current across the electrodes.
The cathodic tool may be made of various conductive metals, but preferably of carbon.
Irrespective of being acidic or alkaline, ordinary electrolytes used for general electrolytic processes may be employed in the present invention. Preferable is a KOH solution with a concentration of 4-6 mM. However, the type and concentration of the electrolyte can be changed according to the material to be processed.
Prior to the preparatory step, the workpiece is preferably cleaned through ultrasonication with acetone and distilled water to remove impurities from the surface of the workpiece.
This preparatory step S10 is requisite for a precise machining process because the volume to be additionally processed owing to the surface tension must be taken into consideration and measured. The volume to be additionally processed owing to the surface tension generated when immersing the workpiece into the electrolyte can be calculated from the following equation:
Vp=xcfx80h(xe2x88x922D2xe2x88x92DoD+3Do2)/15
wherein, Vp is a volume which is additionally processed owing to the surface tension generated when the workpiece is dipped into the electrolyte, represented in mm3; h is the practical length of the workpiece which is in contact with the electrolyte owing to the surface tension, represented in mm;
D is the diameter of the workpiece""s portion processed, which changes as the electrochemical machining process proceeds, represented in mm; Do is the original diameter of the workpiece""s portion to be processed, represented in mm.